Menstrual cycle shows changes in ovary and uterus simultaneously. It begins with menstruation i.e. bleeding phase. It continues for five days. Menstruation means shedding of endometrial lining from uterus. This occurs due to decrease in levels of ovarian hormones: Estrogen and Progesterone. These hormones decrease because in the ovary the corpus luteum of previous cycle start disintegrating. Thus degeneration of corpus luteum causes the ovarian hormones to decrease and thus endometrium degenerates leading to bleeding phase.